The present invention relates to carbon fiber reinforced carbon composites which are excellent in strength and friction properties, and a method for preparing the
Generally, carbon fiber reinforced carbon composites (referred to as "C/C composites" hereinafter) are prepared by impregnating into or mixing with long or short carbon fibers such as PAN based carbon fibers, pitch based carbon fibers and rayon based Carbon fibers, etc. thermosetting resins such as phenol resins, furan resins, etc. or thermoplastic resins such-as pitches, etc., hot-molding the impregnated or mixed material, baking the thus obtained products at a temperatures of 600.degree. to 2500.degree. C. under non-oxidizable atmosphere, and then densifying them.
As the methods for preparing C/C composites composed of short carbon fibers, there may be mentioned a method in which a solvent and a carbon fiber are mixed by a mixer, the resultant mixture is matted by a screen, and then pressure is applied to the thus-obtained mat for preparing a porous carbon electrode (Japanese Patent Application Laid-Open (KOKAI) Nos. 54-41295 and 57-129814); a method in which a solvent and carbon fibers having different length are mixed in a mold by ultrasonic vibration (Japanese Patent Application Laid-Open (KOKAI) No. 58-30537); a method in which a short fiber in the form of mat is laminated, and after impregnating with resins or pitches, the laminate is molded (Japanese Patent Application Laid-Open (KOKAI) No. 62-119288); a method in which a short Carbon fiber is dispersed uniformly by applying a beating-treatment in a solvent, and then the solvent is removed, thereby obtaining an aggregate of carbon fiber orientated in a random direction (Japanese Patent Application Laid-Open (KOKAI) No. 62-96364); and a method in which a short fiber and a solvent are supplied into a frame having a large number of pores in the bottom for drawing solvents, and after dispersing uniformly, the solvent is drawn to form a preform (Japanese Patent Application Laid-Open (KOKAI) No. 1-176273).
Since by the method described in Japanese Patent Application Laid-Open (KOKAI) No. 62-119288, since a matted short fiber is laminated and resins or pitches are impregnated therein, each sheet of the matted short fiber is insufficiently impregnated with resins or pitches. As a result, it is difficult to mold comparatively thick samples, large void pores (holes) are easily made between carbon fibers or between the laminated carbon fibers layers, and thus-obtained composites are inferior in strength. Then, there is a problem that irregular distribution of carbon fibers easily arise during preparation of heavy preforms in the method described in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-96364 and 1-176273. Further, there are problems that by other methods, since fibers are oriented in the pseudo-three-dimensional direction, irregular distribution of carbon fibers and local void pores are easily produced, the strength thereof is uneven, and even when the fibers are used as friction materials or sliding materials, uniform characteristics cannot be obtained.
As a result of earnest research to overcome these problems, it has been found that by subjecting short carbon fibers to dry or wet fibrillation to prepare sheets oriented in the two-dimensional random direction, impregnating resins or pitches into the sheets, laminating and molding the impregnated sheets, and then subjecting to baking and densification, the thus-obtained C/C composites have good mechanical strength and are excellent in friction properties. The present invention has been achieved on the basis of this finding.